Sewing thread has been typically coated with monomeric substances to aid in sewing, and to protect the thread. These coatings, or finishes, include such materials as silicones, fatty acid derivatives and phosphate esters. In the case of heavy industrial sewing, such monomeric coatings do not provide sufficient protection and, accordingly, polymer coatings have been used.
In addition to abrasion resistance, polymeric coatings provide actual bonding of the individual strands of the thread. This bonding provides a smoother thread surface and allows the thread to pass easily through the eye of the sewing needle. In addition, bonding allows thread to be made of filament bundles having lower levels of twist. Lower levels of twist result in lower costs for thread.
A particularly useful class of polymeric coatings which provides abrasion resistance as well as bonding are the nylon terpolymers. These terpolymers are readily soluble in lower alcohols. The coating technique is generally practiced by passing the thread through an alcohol solution of the terpolymer and subsequently removing the solvent by heat. Threads coated with these nylon terpolymers are highly resistant to abrasion, are well bonded and are suitable for many sewing applications.
In more demanding sewing applications, ordinary polymeric coatings may not meet the stringent requirements for a bonded thread. High speed sewing into heavy material, for instance, may result in fibrillation of bonded thread so that individual strands which may make up the bonded bundle of thread can separate when the thread is subject to sewing stresses. This fibrillation results in poor stitches and is generally unacceptable.
In order to improve the bond strength, a cross-linkable bonding agent has been used heretofore for thread coating. Cross-linking improves the thread to binder adhesion as well as the bulk strength of the bonding material itself. A suitable cross-linking system makes use of "type-8" nylon which is an alkoxyalkylated nylon 66. Type-8 nylons, which are dissolved in suitable solvents, are applied to thread in the manner described hereinabove. Cross-linking of the "type-8" nylon is catalyzed by a suitable acid and takes place as the solvent is removed from the polymer coating. Nevertheless, the manufacture of "type-8" nylon resins is complex and expensive. This drawback has hindered the development of threads with a cross-linked coating.
In addition to using "type-8" nylon, cross-linking of nylon terpolymers has been achieved by the use of epoxy resins, phenol formaldehyde resins and melamine formaldehyde resins. These resins, in solutions with the nylon terpolymers, react with the terpolymers at elevated temperatures. The effect is to cross-link the nylon resin which results in a harder, more durable thread coating.
While the epoxy resins result in highly cross-linked coatings, they require longer cure times than either the "type-8" nylon or the phenol formaldehyde or melamine formaldehyde resins. Longer cure times result in slower coating rates and at times, tacky, uncured coatings. Phenol formaldehyde resins and melamine formaldehyde resins result in highly cross-linked coatings and the curing takes place in a timely manner. However because of residual formaldehyde in these resin formulations, environmental concerns have been raised so that there has been an understandable reluctance to use this system in coating applications.
Another means for minimizing breakage of multifilament sewing threads is provided in U.S. Pat. No. 3,823,031, which describes the use of a thermoplastic segmented copolyester elastomer as a bonding agent coating for nylon and polyester multifilament threads. By bonding together the individual filaments, untwisting is minimized which helps to eliminate breakage.